In one type of thermal printer which prints colored images, a donor contains a repeating series of spaced frames of different colored heat transferable dyes. In such apparatus, the donor is disposed between a receiver, such as coated paper, and a print head formed of, for example, a plurality of individual heating resistors. When a particular heating resistor is energized, it is heated and causes dye from the donor to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the donor.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to teach heating element, yielding a variable dye density image pixel in the receiver.
In the conventional thermal dye transfer printer using a plurality of individual heating resistors, the thermal time constant of the resistors are quite long so that the rate of dissipation of heat is slow. As a result, the printing speed and the image contrast are limited. Alternatively, a very high power laser can be used in thermal dye transfer printing. However, the use of such a high power laser is bulky, expensive, and difficult to operate and maintain. Also, the energy required for dye transfer is very high. This high energy requirement limits printing speed.